This invention relates to coaxial electrical cables, and more particularly, to flexible coaxial cables for carrying microwave signals in the gigaHertz range with extremely low attenuation and low radiation losses.
In conventional coaxial cables, a center conductor is surrounded by a dielectric which in turn is surrounded by an outer conductive shield serving as an outer conductor generally coaxial with the center conductor. This outer shield is conventionally formed by a braid of electrical wires and in some cables a second braided shield surrounds the first and the composite is called a double shield braid. Such conventional cables have been found suitable for most applications but are totally unsuited for the very highest frequency applications, for example, in the gigaHertz range, because the attenuation losses of such conventional coaxial cables often are totally unacceptable for use in the gigaHertz (GHz) applications.
Among other problems with these conventional braid shielded coaxial cables are that the braid itself provides windows or openings through which electrical energy leaks or radiates from the cable. This occurs even if multiple braids or layers are employed because the radiation travels between the layers and leaks out through the windows in the outer braid. Also, the flexing of the cable at very high frequencies tends to generate "noise" by the rubbing contact between braids.
In U.S. Pat. No. 4,408,089 of my father the aforesaid problems were addressed in the form of an extremely low attenuation low radiation loss flexible coaxial cable for handling microwave energy in the GHz frequency range. In that patent a flexible dielectric medium which covered a center conductor was surrounded by a plurality of longitudinal, parallel, contiguous conductive strands with a slight helical lay which in turn were surrounded by means to hold them in place, including an outer jacket of flexible impermeable material such as plastic. The coaxial cable of that patent provides superior performance with respect to attenuation loss, leakage, and other properties as compared with conventional braided coaxial cables for microwave work.
The dielectric utilized in the aforesaid patented coaxial cable was a high density PTFE (polytetrafluroethylene).
It would be desirable to use low density dielectric material containing many tiny air pockets filling the region between the central conductor and outer conductor in order to further reduce the attenuation loss. In the GHz range such low density PTFE dielectric exhibits lower losses than high density solid PTFE material. However, low density dielectric is very difficult to use where the accurate terminations are required, because of its own mechanical unstability. This low density dielectric is both "mushy" and "springy", making it difficult to trim accurately in preparation for the attachment of a connector to the end of the cable. Accordingly, using the low density PTFE material for the dielectric causes a severe problem when the shield is removed because no retaining force remains to hold the dielectric in shape. Accordingly, attempting to make an external connection to such a cable becomes a frustrating, unmanageable, unpredictable task, which often ends in failure.